A schematic stack structure of a general nitride semiconductor light emitting device and a fabrication method thereof will now be described.
FIG. 1 is a sectional view of a general nitride semiconductor light emitting device.
Referring to FIG. 1, a conventional nitride semiconductor light emitting device includes a substrate 101, a buffer layer 103, an n-GaN layer 105, an active layer 107 and a p-GaN layer 109.
Herein, in order to minimize the occurrence of crystal defects due to differences in the lattice constants and the thermal expansion coefficients of the substrate 101, for example, sapphire substrate, and the n-GaN layer 105, a GaN-based nitride or an AlN-based nitride having an amorphous phase at a low temperature is formed as the buffer layer 103.
The n-GaN layer 105 doped with silicon at a doping concentration of 1018/□ is formed at a high temperature as a first electrode contact layer. Thereafter, the growth temperature is lowered and the active layer 107 is formed. Thereafter, the growth temperature is again elevated and the p-GaN layer 109 doped with magnesium (Mg) is formed.
The nitride semiconductor light emitting device having the aforementioned stack structure is formed in a p-/n-junction structure which uses the n-GaN layer 105 as the first electrode contact layer and uses the p-GaN layer 109 as the second electrode contact layer. A second electrode material formed on the second electrode contact layer is limited depending on a doping type of the second electrode contact layer. In the conventional art, in order to decrease the contact resistance between the second contact material and the p-GaN layer 109 having a high resistance component and enhance the current spreading, a thin transmissive resistance material of a Ni/Au alloy is used as the second electrode material.
In the technique of fabricating the conventional art p-/n-junction light emitting device, in order to suppress the crystal defects occurring due to differences in the lattice constants and the thermal expansion coefficients of the sapphire substrate and the n-GaN semiconductor, a low temperature GaN buffer layer or AlN buffer layer can be used to obtain a GaN semiconductor having the crystal defect density of ˜108/□. However, in order to increase the optical power of the light emitting device and enhance the reliability of the light emitting device against ESD (electro-static discharge) or the like, it is required to grow a GaN semiconductor having a much lower crystal defect density.
To solve the above problems, a variety of growth techniques, such as a ‘lateral overgrowth’ or ‘pendeo-growth’ using insulator or refractory metal have been employed to decrease the crystal defect density to at least ˜107/□.
However, these growth techniques have a drawback such as the complexity in the fabrication technique. Also, these growth techniques can use a GaN substrate to effectively suppress the crystal defects, but are required with continuous research and development for mass production in terms of technique or price.
Occurrence reason of the above crystal defects and problems caused by the crystal defects will now be described in more detail. First, in the case of low temperature amorphous GaN buffer layer or low temperature amorphous AlN buffer layer, when the amorphous layer is recrystallized at a high temperature, it forms a ‘poly-like’ crystal phase. Therefore, an initial GaN semiconductor has a very rough surface state and a bad flatness, but as the growth continues, a vertical growth is preceded and then a two-dimensional growth is preceded to obtain a good quality of nitride semiconductor. At this time, in the vertical growth corresponding to the initial stage of the growth, crystal defects are generated at a boundary of a fusion with a GaN island. The crystal defects are generated in various types propagated to a surface of the light emitting device, such as ‘threading dislocation’, ‘screw dislocation’, ‘line dislocation’, ‘point defect’, or ‘mixture’, which badly influences the device reliability.
Especially, in the case of the sapphire substrate, the ‘threading dislocation’ is propagated to the surface of the light emitting device. During the propagation, the ‘threading dislocation’ passes through the active layer emitting light and accordingly it later serves as a current path for leakage current or the like. For example, when a high voltage such as ESD or the like is instantly applied, the active layer is destroyed or the optical power is lowered. The above problems provide fundamental reasons badly influencing the reliability.
Due to the above reasons, to effectively enhance the optical power and reliability of the light emitting device, a crystal growth method that can fundamentally minimize the crystal defects propagating from the sapphire substrate is essentially required.